Towards artificial leaves for solar hydrogen and fuels from carbon dioxide

Abstract

The development of an "artificial leaf" that collects energy in the same way as a natural one is one of the great challenges for the use of renewable energy and a sustainable development. To avoid the problem of intermittency in solar energy, it is necessary to design systems that directly capture CO2 and convert it into liquid solar fuels that can be easily stored. However, to be advantageous over natural leaves, it is necessary that artificial leaves have a higher solar energy-to-chemical fuel conversion efficiency, directly provide fuels that can be used in power-generating devices, and finally be robust and of easy construction, for example, smart, cheap and robust. This review discusses the recent progress in this field, with particular attention to the design and development of 'artificial leaf' devices and some of their critical components. This is a very active research area with different concepts and ideas under investigation, although often the validity of the considered solutions it is still not proven or the many constrains are not fully taken into account, particularly from the perspective of system engineering, which considerably limits some of the investigated solutions. It is also shown how system design should be included, at least at a conceptual level, in the definition of the artificial leaf elements to be investigated (catalysts, electrodes, membranes, sensitizers) and that the main relevant aspects of the cell engineering (mass/charge transport, fluid dynamics, sealing, etc.) should be also considered already at the initial stage because they determine the design and the choice between different options. For this reason, attention has been given to the system-design ideas under development instead of the molecular aspects of the O2- or H2-evolution catalysts. However, some of the recent advances in these catalysts, and their use in advanced electrodes, are also reported to provide a more complete picture of the field. A match made in leaves: Recent progresses in the field of the design of artificial leaves, particularly from the system integration point of view, and of its key elements (catalysts, electrodes, membranes, sensitizers), particularly from the biomimetic perspective, are discussed. Emphasis is given to the challenge of producing liquid fuels from the reduction of CO2 using the protons/electrons produced from water oxidation. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.